A widely used variable speed belt drive consists of a pair of sheaves and a belt interconnecting the sheaves in which one of the flanges on at least one of the sheaves is mounted on an axially movable hub and is adjustable toward and away from the other flange. The hub is mounted on a shaft, and a keying device, such as a key and key ways, interconnects the flange, hub and shaft and prevents relative rotation therebetween. The adjustable sheave is varied by an operator or in response to a control signal, and the other sheave automatically adjusts itself to accommodate the adjustment made in the first sheave. In a modification of this basic type, one of the sheaves is variable and the other is moved relative to the adjustable sheave to compensate for variations in the effective belt length when the adjustment in the speed is made.
Various types of mechanisms and systems have been used for adjusting the movable flange of the variable pitch sheave or sheaves. These include mechanical types, manually operated or power driven, or hydraulic types. In both the mechanical and hydraulic types of prior adjustment mechanisms, the movable flange is keyed to the shaft and is adapted to move axially on the shaft in response to changes in the setting on the adjustment mechanism therefor. These adjustment mechanisms and systems function satisfactorily under normal operating conditions while the drives are in good condition, but they often become unreliable or inoperable from fretting and scoring of the shaft and keys. In order to overcome this difficulty, plastic bushings disposed between the shaft and the hub of the movable flange, and in some instances along the keys, have been used; however, the basic reason for the adverse condition developing along the keys is not eliminated by these bushings, although the problem may be somewhat alleviated. Further, the hydraulic system for adjusting the movable flange is often relatively complicated and difficult to maintain in optimum operating condition.
A more recent development, which is disclosed in copending applications Ser. No. 183,627, filed Sept. 2, 1980, and Ser. No. 191,316, filed Sept. 26, 1980, involves the use of a fluid operated device, consisting of a flexible torsion and fluid pressure device of annular shape disposed around the constant speed shaft of the variable speed belt drive adjacent the movable flange thereof, for performing the dual function of controlling the axial position of the movable flange and, at the same time, of coupling the movable flange to the shaft on which it is mounted. A system which is responsive to some predetermined operating condition or a preset speed, controls the operation of the fluid operated device to move the flange to the position required to obtain the desired performance of the drive. The fluid operated device is so constructed and designed that, when coupled to the shaft and movable flange, the torque is transmitted between the shaft and flange without a key or other element directly connecting the two components, and the device is capable of use merely as the coupling element between the shaft and flange or merely as a fluid controlled element for adjusting the movable flange axially on the shaft.